Fuels represent a crucial energy supply and an important revenue source. Based on their provenience and quality (e.g., different grades or types of fuel), fuels can be differentially priced, such as taxed fuel and subsidized fuel or tax-free fuel; kerosene; diesel fuel; low-octane gasoline; high-octane gasoline; etc. Fuels can be differentially priced for a variety of reasons. In some countries, liquid fuel, such as diesel fuel, kerosene, and liquefied petroleum gas, is subsidized or sold below market rates to provide more widespread access to resources. Fuel can also be subsidized to protect certain industry sectors, such as public transportation.
Fuel adulteration is a clandestine and profit-oriented operation that is conducted for financial gain, which operation is detrimental to the rightful owner. Sometimes, fuels can be adulterated by mixing together fuels from different sources to obscure the origin of one or more of the fuels. Other times, adulterated fuels can be obtained by mixing higher priced fuel with lower priced fuel (e.g., lower grade fuel) or adulterants such as solvents. In some cases, subsidized fuel can be purchased and then re-sold, sometimes illegally, at a higher price. For example, subsidized fuel can be purchased and then mixed with other fuel to disguise the origin of the subsidized fuel.
Fuel markers can be added to fuels to establish ownership and/or origin of fuel. Fuel adulteration can be assessed by determining the presence and concentration of fuel markers in a fuel sample via a variety of analytical techniques, such as fluorescence spectroscopy, gas chromatography (GC), mass spectrometry (MS), etc. Fuel markers can interact with their immediate environment (e.g., matrix), such as fuel, solvent, etc., surrounding the marker, and the effect of the matrix can hinder the analysis of a fuel sample for determining whether a fuel is adulterated or not.
The variable nature of fuel products renders them a challenging medium for fluorescence-based analysis. Changes in fluorescence absorbance and emission bands result from fluctuations in the structure of the solvation shell around a fluorophore. Moreover, spectral shifts (both bathochromic and hypsochromic) in the absorption and emission bands are often induced by a change in solvent mixture or composition; these shifts commonly referred to as solvatochromic shifts, are experimental evidence of changes in the solvation energy. In other words, when a fluorophore is surrounded by solvent molecules, its ground state and excited state are more or less stabilized by fluorophore-solvent interactions, depending on the chemical nature of both the fluorophore and solvent molecules.
Generally, measurement sensitivity of fluorescent markers in fuels using fluorescence spectroscopy is blunted by poor measurement precision across samples due to the complex interaction of marker and fuel fluorescence across a wide spectrum of fuel matrices/formulations. Sample to sample variation in fluorescence measurement quality results in poor overall marker quantitation accuracy, which limits the extent to which fluorescence-based portable analyzers may be used in providing real-time actionable insights into fuel adulteration and/or diversion activities. Conventional analytical approaches to determine fuel adulteration and mitigate matrix effects have significant limitations that preclude their utility in fuel authentication.
Further, accurate estimation of fluorescent markers in fuels using fluorescence spectroscopy requires a thermally controlled measurement environment because of the influence of temperature on sample fluorescence emission. Generally, there are three critical components (e.g., an excitation source such as a laser-based excitation source, a sample, and a detector such as a light dispersion module) that exhibit sensitivity to temperature to varying degrees in fluorimetric measurements. Consequently, thermo-electric cooling (TEC) modules are a critical part of bench top spectrometers. The use of fluorescence-based fuel monitoring devices under field deployment conditions requires a degree of portability that precludes the use of thermally controlled units. The incorporation of hardware components that mitigate the effects temperature, such as thermo-electric cooling (TEC) modules for example, does not only add to the size, weight and cost of the spectrometer, but also significantly increases the power consumption of the instrument and the consequent need of a sufficiently powerful (and thus relatively short-lived) battery for field testing use. Thus, there is an ongoing need to develop and/or improve methods for detecting fuel markers.